This invention relates to an adhesive composition for printed wiring boards wherein necessary wiring patterns are formed by electroless plating, and excellent in resistance to electrolytic corrosion and storage stability.
Printed wiring boards wherein necessary wiring patterns are formed by electroless plating are obtained by an additive process using an insulating substrate having thereon an adhesive on necessary portions and forming thereon wiring patterns by electroless plating. For example, an additive process-produced printed wiring board is obtained by forming an adhesive layer containing a catalyst for electroless plating on a surface of an insulating substrate made from a resin containing a catalyst for electroless plating, masking portions other than circuit portions with a resist, chemically roughening exposed portions of the adhesive layer with an acidic etching solution such as chromic acid-sulfuric acid, neutralizing, washing with water and conducting electroless plating.
As the adhesive for electroless plating there are used compositions comprising acrylonitrile-butadiene rubber or acrylonitrile-butadiene rubber having carboxyl groups at both ends, excellent in adhesiveness to deposited copper, as a major component, and a thermosetting resin such as heat-resistant alkyl phenol resin, epoxy resin, or the like, and if necessary, an inorganic filler for helping reinforcement of adhesive coating and chemical roughening, as disclosed in Japanese Patent Examined Publication Nos. 48-24250, 45-9843, 45-9996, 55-16391, and 52-31539, and Japanese Patent Unexamined Publication No. 51-28668. Further, as the adhesive for electroless plating without conducting seeding, there is proposed in U.S. Pat. No. 4,522,850 a composition comprising acrylonitrile-butadiene rubber or acrylonitrile-butadiene rubber having carboxyl groups at both ends, excellent in adhesiveness to deposited copper, as a major component, a thermosetting resin such as heat-resistant alkyl phenol resin or epoxy resin and a filler having a catalytic property for electroless plating. But these adhesives are insufficient for conducting the additive process. Particularly, when a conventionally used palladium-containing catalyst for electroless plating is mixed with the adhesive for conducting the additive process without seeding, the catalytic property is lost in a short time by the reaction of palladium and the carboxyl group.
On the other hand, with a recent demand for miniaturization and lighter weight of electronic appliances, a higher density of wiring has also been required for printed wiring boards. As a result, distances between wiring conductors become narrower, which results in causing a problem of readily bringing about electrolytic corrosion wherein conductors are migrated by various treating solutions remaining on the surface or interior of an insulating substrate supporting the conductors and activated by an electric field applied between conductors. Therefore, it is impossible to make the distance between conductors 0.15 mm or less, resulting in difficulty in realizing higher density of wiring.
The electrolytic corrosion seems to be caused by ionic impurities contained in the adhesive layer formed on the substrate or treating solutions specific for producing printed wiring boards by the additive process, these ionic impurities and treating solutions being activated by an electric field applied between conductors under circumstances of high temperatures and high humidity, resulting in migrating copper.